My doc says the sleep study also checks for rls. what would cause someone to have rls?

Not exactly: Rls is condition that you have when you're awake. Sleep study will not assess RLS any more than a friend sitting next to you on the couch assesses rls. What sleep studies can diagnose is something called plmd (periodic limb movement disorder). This can be a related disorder to RLS but these are twitches or movement that occur in a stereotyped, repetitive behavior during the night while asleep.

Genes, low iron: While most cases are idiopathic (no clear cause), there are some genes that have been implicated in causing rls. People with iron deficiency and neuropathy are also more prone to develop rls, and treating the underlying disease often causes the RLS to get better.